This invention concerns a carrier recovery network for demodulating a high definition television signal, e.g., of the VSB-modulated type adopted for use in the United States.
The recovery of data from modulated signals conveying digital information in symbol form usually requires three functions at a receiver: timing recovery for symbol synchronization, carrier recovery (frequency demodulation to baseband), and channel equalization. Timing recovery is a process by which a receiver clock (timebase) is synchronized to a transmitter clock. This permits a received signal to be sampled at optimum points in time to reduce slicing errors associated with decision-directed processing of received symbol values. Carrier recovery is a process by which a received RF signal, after being frequency down converted to a lower intermediate frequency passband (e.g., near baseband), is frequency shifted to baseband to permit recovery of the modulating baseband information. Adaptive channel equalization is a process by which the effects of changing conditions and disturbances in the signal transmission channel are compensated for. This process typically employs filters that remove amplitude and phase distortions resulting from frequency dependent time variant characteristics of the transmission channel.
In accordance with the principles of the present invention, a carrier recovery network produces a demodulated signal in response to a pilot carrier component of the received signal and a locally generated phase correction control signal representing an unwanted phase offset of the pilot signal transmitted with the main data signal.
In a preferred embodiment, a phase offset in a demodulated signal caused by a phase offset of a pilot signal employed in the carrier recovery process is removed before equalizing the demodulated signal. The carrier recovery network uses two rotators (multipliers), both responsive to received I, Q signals to be demodulated. One rotator is associated with a phase control loop that responds to the pilot component. The other rotator also responds to a combined signal produced by combining a signal derived from the phase control loop with an offset signal representing an estimate of undesired phase distortion, such as multipath (xe2x80x9cghostxe2x80x9d) distortion in the pilot signal.